Mobile network coverage is not universal and tends to be focused around populated areas and major routes between populated areas. When a mobile device moves out of these areas, it may not receive cell-tower coverage and thus be unable to communicate.
The inability to communicate may be problematic when a mobile device requires the transmitting of an important message. Such important messages may, for example, include emergency situations where the mobile device is trying to provide a distress call to an appropriate first responder such as the police department, medical emergency personnel, fire department, among others.
Furthermore, in an emergency situation the mobile device may stop moving and thus be unlikely to re-establish cell-tower coverage within a reasonable time.